1. Field of the Invention
The present invention relates to flash fixing apparatus for fixing toners on a medium by a flashlight and a printer using the same, and more particularly flash fixing apparatus for fixing a high resolution toner image with reduced non-uniformity of halftone image density and a printer using the same.
2. Description of Related Arts
In a printer for forming a toner image using the electrophotographic method or the like, an image formed of powder toner is produced on a print medium. The image is then fixed by fusing the powder toner. Energy must be applied to the print medium to fix the toner image.
In a high-speed printer, a non-contact type fixing method is employed for applying the fixing energy. The non-contact type fixing method is suitable for fixing a toner image in a high-speed printer because the method enables to apply high fixing energy without affecting a print medium to carry.
As this non-contact type fixing method, there has been employed a flash fixing method using flashlight emitted from a flash lamp. In this flash fixing method, fixing is performed on each predetermined area on the print medium by flashing the flash lamp at predetermined intervals synchronously with carrying the print medium.
In such a flash fixing method, it is efficient to fix toner images on the predetermined area of the print medium by one-time flash. However, because the flash energy distribution of single flashlight is not uniform, it is performed to generally superpose a plurality of flashes in order to obtain uniform flash energy distribution. The fixing characteristic depends on both the light energy distribution and the range of superposition area. There have been proposed various arts to obtain a desired characteristic.
A first prior art is shown in FIG. 26, in which a trapezoidal reflection plate 112 is provided around a flash lamp 111 to produce light energy distribution xe2x80x98exe2x80x99 onto a print medium 106. The produced energy distribution xe2x80x98exe2x80x99 is configured so that 70%-80% of the total irradiation energy is concentrated onto a center zone xe2x80x98axe2x80x99 of the irradiation area A. Providing that the length of the area into which 70% of the total irradiation energy is concentrated is defined as a fixing width W, the relation between moving velocity V of a continuous medium and flash frequency f of the flash lamp 111 is defined by formula (1) shown below.
V/f=W/nxe2x80x83xe2x80x83(1) 
In this formula, it has been proposed to set xe2x80x98nxe2x80x99 within a range of 1.2-1.8, preferably 1.3-1.7 (for example, as disclosed in the official gazette of Japanese Patent No. 2870705.)
Here, V/f denotes a moving distance of the continuous medium in a time between two flashes (in other words, an area length allotted for one flash.) This distance becomes shorter than the fixing width W by setting xe2x80x98nxe2x80x99 to the above-mentioned value. Accordingly, the fixing width is set so that superposition is always existent. This produces the light energy distribution E shown in FIG. 27 against the continuous medium. Thus prevention of non-uniform fixing is intended.
Now, according to a second prior art, such a reflection plate 112 as shown in FIG. 29(B) is provided around the flash lamp 111, so as to obtain a flat flash energy distribution characteristic at the center of the lamp, as shown in FIG. 29(A). Referring to FIGS. 28(A), 28(B), provided that a fixable area L2 is the width of the flash lamp 111, a half width L1 of the aperture of the reflection plate 112 is defined by the following formula (2), using the relation between moving velocity V of the continuous medium and flash period T of the flash lamp 111.
V/Txe2x89xa6L1xe2x89xa6V/Txe2x88x92L2/2xe2x80x83xe2x80x83(2) 
Namely, it has been proposed that the superposition width produced by the superposed flashes is set between the range of L1 (in maximum) and L1xe2x88x92L2/2 (in minimum) (for example, as disclosed in the official gazette of Japanese Unexamined Patent Publication No. Hei-6-308852.)
Such prior arts disclose method for suppress the variation in the flash energy distribution so as to prevent variation of the toner-fixing rate. In other words, the prior arts are based on the concept that the flash energy is more than sufficient for toner-fixing onto the entire area of a continuous medium, and that excess energy is prevented so that toner burst is not produced.
However, in recent years, it has been required to print halftone images, in addition to characters, and to print halftone images with high resolution. Grayscale is represented by the number of black dots in a predetermined area such as an example shown in FIG. 30, in which dot printing having alternating one xe2x80x98onxe2x80x99 dot and one xe2x80x98offxe2x80x99 dot repetitively in the sub scanning direction. In this example, as shown in FIG. 31, each dot size is larger in case of lower resolution (for example, 240 dpi), or smaller in case of higher resolution (for example, 600 dpi). Further, when the flash energy is applied, the toners within a dot are fused and the fused toners overflow outside the contour of the dot of interest. The size of the toner-overflowed area depends on the flash energy. That is, the overflowed area is relatively small when the flash energy is small, while the overflowed area is relatively large when the flash energy is large.
The difference of such overflowed areas is not so prominent in case of a relatively low resolution of a large dot in order of 240 dpi. However, in case of high resolution in order of 600 dpi, because a dot size is less than half in dot size compared to 240 dpi, the difference of dot diameter resulting from the difference in the overflowed area in the fixed image becomes prominent. In particular, in case of a halftone image, original halftone images having the identical grayscale look as if these images have different halftones.
In the conventional arts, it is intended to apply flash energy sufficient for fixing toner, and to suppress toner burst in case energy more than required for fixing is applied. There has not been considered non-uniformity of image density possibly produced by the fixing using flash energy.
For example, according to the conventional method for producing a uniform flash energy distribution using superposed flashes, there is obtained the density (by outputs of a densitometer) shown in FIG. 32 after fixing the print pattern shown in FIG. 30. As can be seen, the density value varies more than 10, producing a prominent density fluctuation. Especially the output decreases at both the center zone of the fixing and the superposition zone, thus producing stripes (banding).
This is caused by the determination of flash energy distribution, fixing width and superposing width from the viewpoint of preventing non-uniform fixing. There has not been considered variation of flash energy more than the fixing energy. Accordingly, as far as employing conventional superposition theory, it is difficult to prevent non-uniformity of print density under high-resolution printing.
Accordingly, it is an object of the present invention to provide a flash fixing apparatus and a printer using the flash fixing unit for preventing non-uniformity of print density, not only non-uniformity of fixing.
It is another object of the present invention to provide a flash fixing apparatus and a printer using the flash fixing unit capable of high resolution printing reducing non-uniformity of print density.
It is still another object of the present invention to provide a flash fixing apparatus and a printer using the flash fixing unit capable of high resolution printing reducing non-uniformity of print density of a halftone image.
It is still another object of the present invention to provide a flash fixing apparatus and a printer using the flash fixing unit capable of reducing both power consumption and non-uniformity of print density.
To attain the aforementioned objects, a flash fixing apparatus according to the present invention includes a flash fixing unit having a flash lamp and a reflection plate disposed to surround the flash lamp excluding at an aperture portion and for reflecting light from the flash lamp to direct toward the aperture portion, and a controller for controlling to flash the flash lamp. Here, the aforementioned flash fixing unit has an energy distribution characteristic produced by one-time flash against the medium, having substantially constant values at a center zone and decreasing values at both a front zone and a rear zone as each position therein becomes farther from the center zone. The controller controls to flash the flash lamp with such a flash frequency that an energy value obtained by subtracting a toner-fixing start energy value from an added value at both the front zone and the rear zone falls within a predetermined range of the value at the center zone.
Further, according to the present invention, a printer includes the aforementioned flash fixing apparatus and an image forming unit for forming a toner image onto a medium.
The present invention is based on a technical idea to make energy distribution (fusion energy distribution) exceeding the fixing start energy flat, instead of providing flatness in flash energy distribution over one-time flash zone and superposition zone. Here, the aforementioned fusion energy distribution affects print density (which depends on the size of toner-overflowed area). Accordingly, it becomes possible to obtain a high-resolution print image with reduced non-uniformity of print density.
According to the present invention, preferably the controller controls to flash the flash lamp with a flash frequency f which satisfies the following formula:
g(x)+gxe2x80x2(v/f+x)xe2x88x92xcex2Hxc2x1xcex1 % 
where, v is feeding velocity, f is flash frequency, H is energy value at the center zone, g(x) is a characteristic at the front zone, gxe2x80x2(v/f+x) is a characteristic at the rear zone, and xcex2 is the fixing start energy.
This enables to produce a high-resolution halftone print image having the density of reduced non-uniformity which an observer hardly identifies.
Further, preferably xe2x80x987xe2x80x99 is used as the aforementioned value xcex1, aiming to extend tolerable range of the flash frequency.
Still further, according to the present invention, preferably the minimum value of flash energy within a flash energy distribution range corresponding to a reflection plate aperture width corresponds to the fixing start energy. This enables to reduce input energy (power consumption).
Further, according to the present invention, preferably the reflection plate shape is structured such that the minimum value of flash energy within the flash energy distribution range corresponding to the reflection plate aperture width corresponds to the fixing start energy. This reflection plate shape enables to reduce input energy.
According to the present invention, preferably the reflection plate is constituted of side reflection portions, a ceiling reflection portion, and a convex portion disposed in the ceiling reflection portion. With this reflection plate shape, input energy can be reduced.
Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings.